Plastics are abundant in sources, low in cost, low in density, high in specific strength, good in insulation, chemically stable, shock-absorbing and wear-resistant, are increasingly widely used in modern industry and daily life.
Plastics are synthetic materials formed by chemical polymerization of monomers that can form polymers with high relative molecular mass. Low molecular compound monomers are transformed into macromolecular substances through polymerization reactions, their atoms form macromolecular structures in the form of covalent bonds, and relative molecular mass is generally not less than 104.

 

Polymer macromolecules are basically long chain structures, with branches formed by added chemical components along main molecular "backbone". Although term plastic is often casually used as a synonym for polymer resin, plastic is usually a mixture whose main component is synthetic resin, and also contains lubricants to help molding, plasticizers to improve mechanical properties, anti-aging stabilizers, fillers to reduce formulation costs, flame retardants to improve physical properties, nucleating agents to improve optical properties, and other additives.

 

Polymer monomers, plastic composition, polymerization process

Generally speaking, plastics are made into final products mainly through physical phase changes such as melting and solidification (for thermoplastics) or chemical reactions (for thermosetting plastics). Following figure shows proportion of various manufacturing methods in the total amount of plastics.

 

 

 

Thermoplastic recycling principle
Thermoplastic molding process is theoretically a physical process that only occurs such as phase change, just like ice turns into water when heated, then water turns into ice again when cooled. Therefore, such plastics should be easy to recycle, such as waste materials (sprues, runners, etc.) in injection molding. However, due to some slight chemical changes (such as oxidative degradation and thermal degradation) that often occur during molding process, performance of second-generation recycled plastic may be different from that of new plastic used for the first time.

 

Thermoplastic recycling principle

 

Microstructure of plastics and effects of heat and cold

 

Comparison of Amorphous and Crystalline

 

 

 

 

Effects of plastic modification and reinforcement

 

Thermodynamic state of plastics
Relationship between physical state of plastics and temperature

 

Glass state: 0~Tg molecules are in a frozen state, hard and brittle, and easily broken when under pressure;
Highly elastic state: Tg~Tf can be deformed by external forces, but not in a molten state, and difficult to shape;
Viscous flow state: Tf~Td can be processed and shaped at will;
Decomposition: Td~Plastics begin to crack, gaseous decomposition products appear, and even reach a charred state.
Rheological properties of plastics
Flow curve and rheological curve of plastic fluids

 

Among plastic melts, viscosity of a few melts such as polycarbonate (PC) and polyamide (PA) is not sensitive to shear strain rate and can be approximated as Newtonian fluids. However, flow of most plastic melts is close to that of pseudoplastic fluids, and they behave as Newtonian fluids only when shear stress is very small or very large. Shear stress and shear rate curve of pseudoplastic fluids behaves similarly to plastic flow at initial stage of bending. Its viscosity and shear rate curve deviates from Newtonian fluid curve and bends downward, and viscosity decreases with increase of shear rate.
Shear Flow

 

Viscosity
Viscosity is ability to resist movement. Melt flow depends on viscosity characteristics, which in turn depends on shear rate, temperature, pressure, and residence time.

 

Plastic molding window
To mold high-quality plastic products, a suitable molding window must be used

 

ABS

Nylons

PA12

PA6

PA66

PBT

PC

PC/ABS

PC/PBT

PE-HD

PE-LD

PEI

PET

PETG

PMMA

POM

PP

PPEIPPO

PS

PVC

SAN

Appearance identification method
Polyethylene (PE), polypropylene (PP), and nylon (Nylon)
All have different bendability, feel hard and waxy to the touch, and make soft keratin-like sounds when tapped. PE-LD is milky white, translucent, soft, flexible and tough before dyeing, and can be slightly stretched. PE-HD is also milky white before dyeing, but it is opaque, hard, and not easy to stretch. PP is white and translucent before dyeing, but it is slightly more transparent than PE-LD, and it is lighter and floats hard. Nylon is slightly yellow before dyeing. Relative density of PE and PP is less than 1, and it is in water. Relative density of Nylon12 (or PA12) is 1.01~1.03, and it is close to being suspended in water.
Polystyrene (PS), acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), plexiglass (PMMA)
All have no ductility, feel rigid to touch, and make a crisp sound when knocked. Difference between PS and ABS is that PS is brittle and easily cracks when bent; ABS is tough and difficult to break when bent. Bending part turns white due to molecular stretching orientation, especially crack. PC is transparent before dyeing, while ABS is light ivory. Original color of PS and PMMA is difficult to identify from appearance. Both have good light transmittance. Even if dyed, their light transmittance, glossiness, hardness, etc. are almost same. PS and most other plastics have a relative density greater than 1 and sink in water.
Combustion observation method
Bakelite (PF) and all other thermosetting plastics - do not soften or melt when heated or burned, but only become brittle and charred.
Polystyrene (PS) and all other thermoplastics - must first undergo a softening and melting process when heated or burned, but different types of plastics have different burning phenomena
Polyethylene (PE): easy to burn, and can continue to burn after leaving fire source. When burning, upper end of flame is yellow and lower end is blue. Combustion is more complete, with very little black smoke. Near flame, plastic has a melting and dripping phenomenon, similar to flow of mineral candle wax, and molten material is rarely stained by smoke. After flame is extinguished, there is a clear smell of burning paraffin.
Polypropylene (PP): Combustion phenomenon is roughly same as PE, and there may be a small amount of black smoke. After flame is extinguished, there is a petroleum smell similar to kerosene.
Nylon (Nylon or PA): Burns more slowly. After removing fire source, if ambient temperature is not too high, or there are metal accessories to take away heat, burning time will not be long, and it will extinguish itself later. Flame color is yellow on the top and blue on the bottom. When near flame, plastic surface will melt and drip, and will bubble. After extinguishing, there is a smell similar to burning wool or nails.
Polystyrene (PS) and other thermoplastics (continued)
Polystyrene (PS) and acrylonitrile-butadiene-styrene (ABS): easy to burn, and can continue to burn after leaving fire source. Combustion is extremely incomplete, flame is yellow, and there are dense black smoke bundles, which escape with smoke and diffuse around. However, carbon bundles of PS are slightly less. When burning, plastic surface near flame softens and is not easy to drip. Surface of PS will bubble, while surface of ABS will not bubble and will be in a charred state. After extinguishing, PS has smell of styrene monomer, and ABS has a very unique odor.
Polycarbonate (PC): Combustion phenomenon is similar to PS, but burning speed is slow. It will slowly extinguish after leaving fire, and smell after extinguishing is fruity.
Plexiglas (PMMA): There is no carbon bundle flying when burning, flame is light blue and top is white, it emits a strong smell of fruity and rotten vegetables after burning.
Polyvinyl chloride (PVC): It is difficult to burn and is easy to self-extinguish after leaving fire source. When burning, flame is yellow above and green at the bottom, sometimes with small yellow-green flames, white smoke and a pungent smell. Melt softens as it burns and can be pulled out into wire.
Plastic selection

Molding shrinkage:
Plastics with small shrinkage (PS, ABS, PC) are easier to achieve dimensional accuracy, while plastics with large shrinkage (PP, PE, POM) are more difficult to achieve dimensional accuracy (mold tolerance is about 116 of product tolerance).
Flow viscosity:
Plastics with large flow viscosity (ABS) are less likely to flow into gap, while plastics with small flow viscosity (PA, POM) are easy to flow into even if gap is very small.
High or low molding temperature
Plastics with lower molding temperatures (PS) are easier to mold and have shorter molding cycles, while plastics with higher molding temperatures (PC) are more difficult to mold and have longer molding cycles.
Is it easy to deteriorate or decompose?
Plastics that are not easy to deteriorate or decompose during molding (PS, PE, PP) are not likely to cause defective products with unstable quality during mass production. Plastics that are easy to deteriorate or decompose during molding cannot be mass-produced if molding conditions are not strictly required (mold can precisely control molding conditions). This problem is particularly serious when using hot runners.